Electron carrier used in photosynthesis

from a molecule in solution to interact with molecules that have been chemically attracted to the surface of the assembly; the second step involves the product of that step reacting with molecules inside the tunnel altogether. "So we have a self-assembling, three-molecule chain, where the viologen is inside, porphyrin is on the outside, and EDTA is ion-paired with the porphyrin out there," Mallouk explained. The assembly worked, in that it propelled the electron into the tunnel in 30 picoseconds, only 10 times slower than with natural photosynthesis. The efficiency of the electron-hole separation was very poor, however, said Mallouk, illustrating Wrighton's emphasis on the importance of back electron transfer pathways.

What 2 electron carriers are used in the citric acid cycle

BI 2: Electron Transport Chain & Chemiosmosis by …

''Natural photosynthesis is a process by which light from the sun is converted to chemical energy," began Mark Wrighton in his presentation to the Frontiers symposium. Wrighton directs a laboratory at the Massachusetts Institute of Technology's Chemistry Department, where active research into the development of workable laboratory synthesis of the process is under way. As chemists have known for many decades, the chemical energy he referred to comes from the breakdown of carbon dioxide (CO2) and water (H2O), driven by photons of light, and leads to production of carbohydrates that nourish plants and of oxygen (O2), which is vital to aerobic organisms. What is not known in complete detail is how this remarkable energy-conversion system works on the molecular level. However, recent advances in spectroscopy, crystallography, and molecular genetics have clarified much of the picture, and scientists like Wrighton are actively trying to transform what is known about the process into functional, efficient, synthetic systems that will tap the endless supply of energy coming from the sun. "Photosynthesis works," said Wrighton, "and on a large scale." This vast natural phenomenon occurring throughout the biosphere and producing an enormous amount of one kind of fuel—food for plants and animals—Wrighton described as "an existence proof that a solar conversion system can produce [a different, though] useful fuel on a scale capable of meeting the needs'' of human civilization. Photovoltaic (PV) cells already in use around the world provide a functional (if more costly per kilowatt-hour)

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Wrighton's presentation, "Photosynthesis—Real and Artificial," was a closely reasoned, step-by-step discussion of the crucial stages in the chemical and molecular sequence of photosynthesis. His colleagues in the session were chosen for their expertise in one or another of these fundamental specialized areas of photosynthesis research. By the end of the session, they had not only provided a lucid explanation of the process, but had also described firsthand some of the intriguing experimental data produced. Douglas Rees of the California Institute of Technology (on the molecular details of biological photosynthesis), George McLendon of the University of Rochester (on electron transfer), Thomas Mallouk of the University of Texas (on the arrangement of materials to facilitate multielectron transfer chemistry), and Nathan Lewis of the California Institute of Technology (on synthetic systems using liquid junctions) all supplemented Wrighton's overview with reports about findings in their own area of photosynthesis research.

Electrons' Role in Photosynthesis ..

McLendon is a chemist specializing in the quantum processes of moving electrons from one molecule to another. Not focusing exclusively on photosynthesis, he usually works with proteins and biological systems, but his laboratory has demonstrated phenomena crucial to all electron transfer systems. The basic physics involves the concept of conservation of energy, which, explained McLendon, shows that an electron's rate of transfer varies with the energy force driving it. Essential to the first step in photosynthesis, this relationship between rate and energy was analyzed theoretically some years ago by Rudy Marcus (1956) at Caltech, who predicted an anomaly that was first confirmed by John Miller at Argonne National Laboratory, and verified subsequently by McLendon and others. Up to a certain level of energy, the rate of electron transfer increases with the force driving it, but the initially proportional relationship changes. After the peak level is reached, additional driving force actually slows the electron down. "A funny thing," said McLendon, "is that you can have too much of a good thing."

Electron Transport Chain of Cellular Respiration - Page 2

Wrighton and others have been able to find and to build multi-component molecules with ''a donor and an acceptor system covalently linked to a light absorber, an assembly,'' he pointed out, "that does indeed resemble the heart of the Z-scheme" found in plants. But their system does not produce energy as efficiently as they would like, because of the timing of the reactions: "In solution the energy-rich molecules lose the stored energy by either intermolecular or intramolecular back electron transfer. In nature, the movement of the carriers away from one another is crucial to high efficiency. The unidirectional movement of electrons in the Z-scheme is a consequence of the components of the charge transport chain, and how they are arranged, both geometrically and energetically," Wrighton explained. Work in his group, he continued, did lead to construction of a complex molecule with all of the functional components, but the photoexcitation test showed that a 10-nanosecond time was required for the donor to deliver the molecule to the acceptor. "This is very sluggish compared to the 4 picoseconds demonstrated [in the natural systems]," Wrighton summarized, "and so one of the challenges is to prepare a molecule that will have more zip than our 10-nanosecond time." Thus chemists explore the quantum world, said Wrighton, narrowing in on several factors that might elucidate the transfer rates of the electrons: "the energetics for electrons, the distance dependence of electron transfer, and the structures of the donor and acceptor and their relationship" in space. George McLendon of the University of Rochester, said Wrighton, "has made important progress in understanding such factors."

path of electron transport in photosynthesis was not ..

The reason has to do with quantum physics at the surfaces. If the semiconductor is in contact with a metal, said Lewis, "you find that the resultant voltages are less than thermodynamics analysis predicts," because of interfacial reactions. A similar problem plagues the interface between two semiconductors of different electrical characters. "If you try to match their surfaces with atomic precision, you will pay a price" to do so, said Lewis, and thus drive up the economic cost of the system. "When you miss at certain spots, those spots become recombination sites," and some of the free charge meant to be collected as electricity is drained into these surface reactions. Using a liquid at the interface obviates both of these problems. First, one can add into the liquid something else that will have a high affinity for the defective sites that could have led to problematic recombination, and can thereby passivate these sites. Second, one can reduce the danger of back electron transfer by choosing a solvent that draws electrons more strongly and increases their forward rate.

and will transfer it into electron carriers …

Figure 2.1 Z-scheme representation of the photosynthetic apparatus showing, on an electrochemical potential scale, components for light absorption, charge transport, and redox processes for oxidation of H2O to O2 and reduction of CO2 and H2O to carbohydrates. (Courtesy of M. Wrighton.)

Photosynthesis & Cellular Respiration Flashcards | Quizlet

to obtain usable electric current. Using insights from "the two systems that really work—photosynthesis and photovoltaics that use semi-conductors—the first question in creating a multistep electron transfer mechanism," according to Mallouk, "is, What is the best way to organize the molecules?"